1. Field of the Invention
The present invention relates to a printed material reader, such as a microfilm reader which searches for a desired frame by using blips recorded or photographed on the printed material, and to a search method therefor.
2. Related Art
There is a well known microfilm search method, wherein blips (rectangular marks) are photographed beside the frames on microfilm and used as reference marks. The blips are read by a blip reader and are counted, and the blip count is used to identify frame addresses when a specific frame is sought. This method is also referred to as a document mark method.
For this method, blips having different lengths, for example, long, medium and short blips, are employed to represent a plurality of classification levels corresponding to a certain criteria. For example, a long blip would represent a high level, a medium blip would represent an intermediate level, and a short blip would represent a low level.
In this blip method, however, it is difficult for the length of a blip to be accurately detected, because such detection has to be performed while the microfilm is being fed at a high speed, and because of a property of the blip reader itself, detected blip lengths may vary. Further, when images on microfilm are reproduced (duplicated), films are placed on top of one another to facilitate the transfer of the images, and as the edges of the blips are not sharply defined, due to the thickness of the film, the length of the blips is therefore slightly changed during duplication.
Since variances in the detected blip lengths occur in this manner, predetermined permissible ranges can be determined and with frames to be searched. However, because of the properties of the blip readers and the properties of the microfilm, the detected lengths of the blips tend to constantly fluctuate. That is, blips on some film tend to shift so that they are longer relative to the permissible ranges, and blips on the other film tend to shift so that they are shorter relative to the permissible ranges.
Therefore, if constant permissible ranges are set, blip detection errors tend to occur and accurate detection is not be possible.